Superconductivity In Electron-doped Cuprates Research Articles

Disorder effects on hot spots in electron-doped cuprates

Antiferromagnetic fluctuations in two dimensions cause a decrease in spectral weight at so-called hot spots associated with the pseudogap in electron-doped cuprates. In the $2\text{D}$ Hubbard model, these hot spots occur when the Vilk criterion is satisfied, namely, when the spin correlation length exceeds the thermal de Broglie wavelength. Using the two-particle self-consistent approach, here we show that this criterion may be violated near the antiferromagnetic quantum critical point when sufficient disorder is added to the model. Static disorder decreases inelastic scattering, in contradiction with Matthiessen's rule, leading to a shift in the position of the quantum critical point and a modification of the conditions for the appearance of hot spots. This opens the road to a study of the interplay between disorder, antiferromagnetic fluctuations, and superconductivity in electron-doped cuprates.

Fine Structure in the Tunneling Spectra of Electron-Doped Cuprates: No Coupling to the Magnetic Resonance Mode

We reanalyze high-resolution scanning tunneling spectra of the electron-doped cuprate Pr(0.88)LaCe(0.12)CuO(4) (T(c) = 24 K). We find that the spectral fine structure below 35 meV is consistent with strong coupling to a bosonic mode at about 16 meV, in quantitative agreement with early tunneling spectra of Nd(1.85)Ce(0.15)CuO(4). Since the energy of the bosonic mode is significantly higher than that (9.5-11 meV) of the magnetic resonancelike mode observed by inelastic neutron scattering, the coupling feature at about 16 meV cannot arise from strong coupling to the magnetic mode. The present work thus demonstrates that the magnetic resonancelike mode cannot be the origin of high-temperature superconductivity in electron-doped cuprates.

Fermi surface evolution and nodeless d-wave superconductivity in electron-doped cuprates

Recent angular resolved photoemission spectroscopy (ARPES) measurements on electron-doped cuprate Nd 2 - x Ce x CuO 4 have revealed the intriguing Fermi surface (FS) evolution with doping. By analyzing the ARPES data, I and collaborators proposed the view that the FS pocket around ( π / 2 , π / 2 ) has not yet formed until doping reaches the optimal value x = 0.15 . Therefore in the underdoped regime, even if the superconducting pairing order parameter is d-wave which vanishes along the diagonal line, the quasiparticle excitation has a finite, i.e., s-wave-like gap due to the absence of the FS across that line. In this paper, I further describe the whole picture for the FS evolution from underdoping to overdoping and the associated property variation from nodeless to nodal d-wave superconductivity. The local density of states is newly calculated for STM measurements.

Evolution of superconductivity in electron-doped cuprates: Magneto-Raman spectroscopy

The electron-doped cuprates Pr_{2-x}Ce_xCuO_4 and Nd_{2-x}Ce_xCuO_4 have been studied by electronic Raman spectroscopy across the entire region of the superconducting (SC) phase diagram. The SC pairing strength is found to be consistent with a weak-coupling regime except in the under-doped region where we observe an in-gap collective mode at 4.5 k_{B}T_c while the maximum amplitude of the SC gap is ~8 k_{B}T_{c}. In the normal state, doped carriers divide into coherent quasi-particles (QPs) and carriers that remain incoherent. The coherent QPs mainly reside in the vicinity of (\pi/2, \pi/2) regions of the Brillouin zone (BZ). We find that only coherent QPs contribute to the superfluid density in the B_{2g} channel. The persistence of SC coherence peaks in the B_{2g} channel for all dopings implies that superconductivity is mainly governed by interactions between the hole-like coherent QPs in the vicinity of (\pi/2, \pi/2) regions of the BZ. We establish that superconductivity in the electron-doped cuprates occurs primarily due to pairing and condensation of hole-like carriers. We have also studied the excitations across the SC gap by Raman spectroscopy as a function of temperature (T) and magnetic field (H) for several different cerium dopings (x). Effective upper critical field lines H*_{c2}(T, x) at which the superfluid stiffness vanishes and H^{2\Delta}_{c2}(T, x) at which the SC gap amplitude is suppressed by field have been determined; H^{2\Delta}_{c2}(T, x) is larger than H*_{c2}(T, x) for all doping concentrations. The difference between the two quantities suggests the presence of phase fluctuations that increase for x< 0.15. It is found that the magnetic field suppresses the magnitude of the SC gap linearly at surprisingly small fields.

Nonmonotonic dx2-y2-Wave Superconductivity in Electron-Doped Cuprates Viewed from the Strong-Coupling Side

Applying a variational Monte Carlo method to a two-dimensional t-J model, we study the nonmonotonic d_{x^2-y^2}-wave superconductivity, observed by Raman scattering and ARPES experiments in the electron-doped cuprates. As a gap function in the trial state, we extend the d-wave form (ext.d) so as to have its maxima located near the hot spots of the system. It is found that, in contrast to the hole-doped case, the ext.d wave is always more stable than the simple d wave in the electron-doped case, and the magnetic correlation of the wave vector (\pi,\pi) as well as the pair correlation is enhanced. These results corroborate spin-correlation-mediated superconductivity in cuprates, recently argued from a FLEX calculation. In addition, we confirm that s- and p-wave symmetries are never stabilized even in the over-doped regime.

Superconductivity in electron-doped cuprates: Gap shape change and symmetry crossover with doping

The Kohn-Luttinger mechanism for superconductivity is investigated in a model for the electron doped cuprates. The symmetry of the order parameter of the superconducting phase is determined as a function of the geometry of the Fermi surface together with the structure of the electron-hole susceptibility. It is found to remain d_{x^2-y^2} wave within a large doping range. The shape of the gap anisotropy evolves with doping, with the maximum gap moving away from (pi ,0), in good agreement with recent experiments. As the shift of the maximum increases, a crossover to d_{xy}-symmetry is found.

Transition temperature and oxygen isotope effect of Nd doped cuprates:a three square well model

Using a three square well model, we investigate the possibility thatin low energy plasmons there could be an additional boson responsible for theattractive force binding the Cooper pairs that lead to superconductivity inlayered electron-doped cuprates. The three square well model is, in principle,characterized by three coupling strengths, the electron-phonon(λph), the electron-plasmon (λpl) and Coulomb screeningparameter (µ*), that estimate the transition temperature (Tc) andoxygen isotope effect coefficient (α). Starting with the three squarewell model within the framework of the Eliashberg theory, the energy gapkernels allow us to visualize the relative interplay of the Coulomb,electron-phonon and electron-plasmon interactions and we correlate the Tcwith these three coupling strengths. For a set of parameters(λph≈1.0, λpl≈0.7 and µ*≈0.18),a Tc of 28 K is estimated for optimally dopedNd1.85Ce0.15CuO4. The present approach also explains the reportedsizeable oxygen isotope effect in electron-doped cuprates. We suggest that theextended attractive force in the proposed three square well schemeconsistently explains the superconductivity in electron-doped cuprates.

Phenomenological model of high- Tc superconductivity: a MCS model

We present a phenomenological model of high- T c superconductivity in hole-doped cuprates: a Magnetic Coupling between Stripes (MCS) model. The MCS model is based upon experimental facts, namely, the presence of (i) stripes, (ii) spin fluctuations, and (iii) two order parameters (for pairing and long-range phase coherence) in hole-doped cuprates. We discuss also the superconductivity in electron-doped cuprates. The MCS model is not applicable to Rhutenate compounds where the superconductivity and magnetism coexist independently.